These days, it's done purely out of tradition. Informally, some people prefer to stay lowercase, for speed and humility - especially when texting in digital slang. Some hyper-modern branding imagery also breaks such rules. A long time ago, capital letters were often made very fancy, and this amplified the difference between cases. Some people say a lowercase 'i' was too easily overlooked or confused with scars & smudges, since it was a small single-letter word, so it was necessary to make it bolder. Hence today, formally and traditionally, it's always capitalised. This is a feature of communication discipline. It's similar to dressing up in a suit & tie, with shiny boots, standing tall and projecting your voice clear & far.

Nobody knows why, not even historians! We have been capitalizing it for hundreds of years. The original reason is lost.

i google it and found below In Old and Middle English, "I" was written as a lowercase letter ("i"), but it looked very small and could easily get lost in handwritten or printed text. To make it stand out and avoid confusion, scribes started writing it in uppercase. Over time, this became the convention.

Great question, I would like to know this as well.